Various systems and devices have been devised to detect, and measure the density of, various fluids, including gas, as well as particulate that may be suspended within these various fluids. For example, many fire detection systems use densitometers to detect the presence of smoke within an environment. As is generally known, smoke is essentially fine particulates, such as carbon, that are generated by combustion and that are suspended in a gas, such as air. Smoke particulates are typically opaque, or at least reflective. Thus, various types of optical densitometers have been developed that detect the presence of smoke, and in some instances the density of smoke, in a gas by measuring the opacity or optical density of the gas.
One particular type of optical densitometer that has been developed includes an optical emitter and an optical receiver, spaced apart from one another in a measurement environment. The optical emitter is configured to emit a relatively constant intensity light beam. The optical receiver is configured to receive the emitted light beam and, upon its receipt, to generate a signal representative of the intensity of the received light beam. If smoke is present within the measurement environment between the optical emitter and optical receiver, the intensity of the emitted light beam may be attenuated before it is received by the optical receiver. Thus, the signal generated by the optical receiver may be representative of this reduction in received light beam intensity.
Although the above-described optical densitometer generally works well, it does suffer certain drawbacks. For instance, the particulate within the measurement environment may adhere to the optical emitter and optical receiver, which can significantly reduce the accuracy of the densitometer. Moreover, if not removed, the amount of particulate adhered to the optical emitter and optical receiver may build up over time, which may have a gradual, continual, yet unpredictable affect on densitometer accuracy.
To alleviate some of the concerns noted above, an optical densitometer configuration was developed in which a source of clean purge gas is used to prevent particulate adherence to the optical emitter and optical receiver. In such a configuration, sometimes referred to as a pinhole purge system, the optical emitter and optical receiver are each housed within individual instrument chambers, which are separated from one another by a measurement chamber. The instrument chambers each include an aperture through which the emitted light beam passes, and each is supplied with a flow of pressurizing air. Thus, the light beam emitted by the optical emitter passes through its instrument chamber aperture, into and through the measurement chamber, and through the aperture in the other instrument chamber, where it is received by the optical receiver. The measurement chamber includes the gas whose density is being measured. This gas is prevented from entering the instrument chambers because the flow of pressurizing air into the instrument chambers exits each instrument chamber via its aperture. Thus, particulate in the measurement chamber cannot adhere to the optical emitter or optical receiver.
Although the above-described pinhole purge system alleviates the problem of particulate adhering to the optical emitter and receiver, it too suffers certain drawbacks. For example, because the pressurized air flows out the instrument chamber apertures along the optical axis between the optical emitter and receiver, the effective optical path length between the emitter and receiver can be shortened, and reduce densitometer accuracy. Moreover, any variation if flow rate of the pressurized air through the instrument chamber apertures can cause variations in the effective optical path length. This latter factor, coupled with the inherent turbulence and swirling effects within the measurement chamber, can result in a relatively unstable effective optical path length, which can adversely affect overall accuracy.
Hence, there is a need for an optical densitometer that addresses one or more of the above-noted drawbacks. Namely, an optical densitometer that is not subject to particulate adhering to its optical instrumentation and/or that does not suffer from variations in effective optical path length and/or accuracy. The present invention addresses one or more of the drawbacks.